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Monday, May 31, 2021

$20 Homebrew Wifi Thermostat with Remote Sensor

I turned on the A/C for the first time this season, only to realize a little while later it was blowing HOT air instead of cold. Could be the reversing valve failed on my A/C unit, but my thermostat has been flaky not to mention super annoying by design so I wasted no time removing it from the wall.

I pulled my home-made, untested thermostat from my workbench, where it has languished for months, vaguely recalling that I left it in a theoretically working state, hung it on the wall, and connected the wires from a hastily retrieved color key. "Hey google, what are the color codes for heat pump wires?" I ask, because my hands are full of wires and pliers and a screw driver and my entomology pins keep ejecting from the shadow box when I wiggle things too much, draping hapless circuits over my hands while I'm trying to work.

I eventually get it all wired up, double check the colors one last time, and ask my wife to turn on the circuit breaker to the furnace until and if I scream. She does. I don't. The LEDs come on, which is a good sign -- first time I've tested the power supply I got cheap on Amazon, to confirm that it does in fact convert 24VAC to something my circuit can survive.

I go to my computer, run my python test script. The thermostat replies, tells me its state. My script registers to receive state change notifications--the thermostat acknowledges. My script shows two temperatures to one tenth of a degree: the temperature at the thermostat in the hallway, and the temperature at the remote sensor in our bedroom upstairs, which I built for a few bucks months ago, plugged in there and forgot about. I tell it to set the cooling temp to 75, using the max of the two sensor temperatures. It acknowledges. A few seconds later the A/C turns on.

Cold Air.

I guess it wasn't the reversing valve after all.

So far, working great, no issues. Now just need to add a UI for it to my home automation software. Bonus: when it's too hot or cold in the middle of the night I can change the temp from bed without having to run all the way downstairs.

Hardware wise, it's virtually identical to my dishwasher controller except with a cheaper power supply (24V input instead of 120) and much easier to wire up (no need for a wiring harness, which was the time consuming part of the dishwasher controller). It was basically an afternoon project to make, and for a ballpark $20 in parts. (See the dishwasher write-up for parts list.)

It lacks a nice 3d printed case and compact circuit board and all that, but I kind of like it. And it lacks a UI, but who needs to stand in the hallway to set their thermostat? Setting it from my phone is just fine.

[Update 2023-02-20: I sketched out a circuit board to have this manufactured more compactly/durably, and it looks like it could be delivered for about $30 each (assembled!) not counting any enclosure, in as little as qty 5-10. So, if you're interested in having a fully-programable, wifi thermostat for $30, let me know. (Especially if you have any hardware chops and can help me sanity check the design before we send it to fab, but even if not.) I probably won't bother just for myself, since the above is working just fine, but it would be fun to have a custom board made to see what's possible.]

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Simon Funk / simonfunk@gmail.com